16,616 research outputs found
Observation of backscattering-immune chiral electromagnetic modes without time reversal breaking
A strategy is proposed to realize robust transport in time reversal invariant
photonic system. Using numerical simulation and microwave experiment, we
demonstrate that a chiral guided mode in the channel of a three-dimensional
dielectric woodpile photonic crystal is immune to the scattering of a square
patch of metal or dielectric inserted to block the channel. The chirality based
robust transport can be realized in nonmagnetic dielectric materials without
any external field.Comment: 16 pages, 5 figure
Symmetry protected topological orders and the group cohomology of their symmetry group
Symmetry protected topological (SPT) phases are gapped short-range-entangled
quantum phases with a symmetry G. They can all be smoothly connected to the
same trivial product state if we break the symmetry. The Haldane phase of
spin-1 chain is the first example of SPT phase which is protected by SO(3) spin
rotation symmetry. The topological insulator is another exam- ple of SPT phase
which is protected by U(1) and time reversal symmetries. It has been shown that
free fermion SPT phases can be systematically described by the K-theory. In
this paper, we show that interacting bosonic SPT phases can be systematically
described by group cohomology theory: distinct d-dimensional bosonic SPT phases
with on-site symmetry G (which may contain anti-unitary time reversal symmetry)
can be labeled by the elements in H^{1+d}[G, U_T(1)] - the Borel (1 +
d)-group-cohomology classes of G over the G-module U_T(1). The boundary
excitations of the non-trivial SPT phases are gapless or degenerate. Even more
generally, we find that the different bosonic symmetry breaking
short-range-entangled phases are labeled by the following three mathematical
objects: (G_H, G_{\Psi}, H^{1+d}[G_{\Psi}, U_T(1)], where G_H is the symmetry
group of the Hamiltonian and G_{\Psi} the symmetry group of the ground states.Comment: 55 pages, 42 figures, RevTeX4-1, included some new reference
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